Ultrasound elasticity imaging is useful for distinguishing tissues having different elastic properties. To perform ultrasound elasticity imaging of tissue in a desired region of interest (ROI), the tissue is excited, or palpated, by an external force, such as by manual palpitation, vibrational devices, and/or ultrasound beams. Alternatively, tissue may be excited by internal forces, such as forces resulting from a heart beat or blood vessel pulsation. Tissue deformation responsive to the excitation may be detected by directing ultrasound beams at the excited tissue and monitoring ultrasonic pulse echoes from the tissue at different times during excitation to obtain strain information related to elasticity of the tissue. Because strain is a function of a derivative of displacement, at least two time separated imaging frames received from the deformed tissue are required for each estimate of strain. Accordingly, elasticity imaging relies on sensing tissue deformation between two imaging frames to obtain strain information.
In conventional ultrasound elasticity imaging, a time interval between imaging frames to be compared for obtaining strain information is typically fixed a constant value. The fixed time interval is typically selected based on a general knowledge about the strain force being applied, such as a frequency and/or amplitude of the force. However, there may be times during elasticity imaging when there is no displacement, or only rigid displacement between frame intervals. Consequently, strain information may be difficult, if not impossible, to obtain. Conversely, when a deformation during a frame interval is too great, the resulting echoes may be insufficiently correlated to enable a reliable strain estimation.